<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        body{
            margin: 0;
        }
        .head{
            height: 100px;
            background-color: #000;
        }
        .main{
            padding-left: 200px;
            padding-right: 250px;
            border: 1px solid red;
            min-height: 500px;
        }
        .fl{
            float: left;
        }
        .main .middle{
            width: 100%;
            height: 500px;
            background-color: skyblue;
        }
        .main .left{
            width: 200px;
            min-height: 500px;
            background-color: yellow;  
            margin-left: -100%;
            /* 百分百相对的是父元素的width而言 */
            position: relative;
            left: -200px;

        }
        .main .right{
            width: 250px;
            min-height: 500px;
            background-color: tomato;  
            margin-right: -250px;
        }

        .footer{
            height: 100px;
            background-color: blue;
        }
    </style>
</head>
<body>
    <div class="head"></div>
    <div class="main">
        <div class="middle fl">middle</div>
        <div class="left fl"></div>
        <div class="right fl"></div>
    </div>
    <div class="footer"></div>
</body>
</html>